This invention relates to electromagnetic transmission lines, and more particularly to short coaxial transmission lines, and methods for using such transmission lines to interconnect RF ports on mutually parallel printed-circuit boards.
FIG. 1 is a simplified perspective or isometric view of a combination 10 of modules, including a main module mated with a plurality of other modules. In FIG. 1, a rectangular, somewhat planar housing 12
contains at least one printed-circuit board lying in the plane parallel to dash line 14, which is to say parallel to that one of the two broad faces, namely near face 12fn. A second housing designated 16a is one of a set 16 of additional housings, illustrated in phantom, which are adapted to be coupled to housing 12 for the flow of electromagnetic energy therebetween by way of coaxial transmission-line structures.
As is well known to those skilled in the art, a coaxial transmission line includes a center conductor lying within, and concentric with, a cylindrical outer conductor. Coaxial transmission lines take many forms, but all are characterized by the flow of electromagnetic energy in the region or space between the center and outer conductors. The generally accepted meaning of a transmission line, as opposed to simple xe2x80x9cwires,xe2x80x9d is that the surge impedance or characteristic impedance of the transmission line tends to be constant along its length, or if the impedance changes along its length, the change is effected in a controlled manner which tends to minimize overall reflection of energy, so that energy introduced into one end of the transmission line is xe2x80x9ctransmittedxe2x80x9d to the remote end with a minimum of loss attributable to mismatch. While coaxial transmission lines may be made to have almost any characteristic impedance, two such impedances are most common. The nominal impedance of 50 ohms is theoretically capable of handling the highest power without voltage breakdown, while the nominal impedance of 75 ohms theoretically provides the least loss per unit length. Military equipments such as radar and other transmitting devices requiring high power thus tend to use 50-ohm transmission lines and associated connectors, while some commercial users such as cable television have, at least in the past, operated at 75 ohms.
In the arrangement of FIG. 1, the coupling between module 16a and a portion of module 12 is provided by coaxial connectors of a set 18a of such connectors, not entirely illustrated in FIG. 1. In FIG. 1, portions of four connectors 18a1, 18a2, 18a3, and 18a4 associated with module 16a are illustrated. It should be understood that this number, and their layout, are illustrative only, and that there may be more or fewer such connectors associated with each module of set 16 of modules.
In order to minimize losses in interconnecting transmission lines, and to minimize the volume occupied by the combined modules 10 of FIG. 1, it is desirable to have each module 16x of set 16 of modules immediately adjacent the near or facing surface 12fn of module 12. In order to accomplish a close spacing, it is possible to use as connectors type GPO interconnects, manufactured by Gilbert Engineering, whose address is 5310 W Camelback Road, Glendale, Ariz. 85301. These connectors are characterized from DC to 40 GHz.
FIG. 2a is a simplified, exploded view of a particular version of the GPO connector, adapted for connecting conductive traces on a printed circuit board to conductive traces on another such board, and FIG. 2b represents a cross-section of a portion of the arrangement of FIG. 1 in its assembled state. For specificity, FIGS. 2a and 2b illustrates connector 18a1 of FIG. 1. In FIG. 2a, 214a represents a portion of the printed circuit board lying within housing 12 in plane 14, and 214b represents another printed circuit board lying within housing 16a. In this particular layout, the two printed circuit boards 214a and 214b lie mutually parallel.
In the arrangement of FIGS. 2a and 2b, both printed-circuit boards are xe2x80x9cstriplinexe2x80x9d boards, in that the transmission lines within each board lie between ground planes. In FIGS. 2a and 2b, printed-circuit board 214a includes a near ground plane 214agn and a remote or far ground plane 214agf. Lying between ground planes 214agn and 214agf are two layers of dielectric or insulating material, a near layer 214am and a far layer 214aif. Lying between insulating layers 214am and 214aif is plane 14 of FIG. 1, and lying xe2x80x9cinxe2x80x9d the plane are a plurality of conductive traces, one of which is illustrated as 214act. It should be understood that the xe2x80x9cplanexe2x80x9d 14 is conceptual, as a plane is dimensionless and therefore cannot contain a three-dimensional object such as trace 214act, however thin.
In FIGS. 2a and 2b, a circular aperture 214aga is defined in ground plane 214agn, thereby exposing the near surface 214ains of near dielectric or insulating layer 214ain. Near the center of aperture 214aga, a conductive pad 214acp is affixed to the near surface 214ains of near dielectric layer 214ain, and is electrically coupled, as by a plated-through via 214av, to an end of the underlying conductive trace 214act. 
Also in FIG. 2a is another printed circuit board 214b, which is similar in its construction to printed circuit board 214a, but which appears somewhat different, since it is seen from the rear rather than from the front. Elements of circuit board 214b corresponding to those of 214a are designated by the same reference numerals, but in the xe2x80x9c214bxe2x80x9d series rather than in the xe2x80x9c214axe2x80x9d series. Thus, printed-circuit board 214b shows a near ground plane 214bgn, a far ground plane 214bgf, a near dielectric layer 214bin, a far dielectric layer 214bif, with a conductive trace 214bct extending therebetween, and ending at the center of an aperture 214bga in near ground plane 214bgn. 
The GPO connector 18a of FIG. 2a includes two surface connector portions 210a and 210b, together with a xe2x80x9cbulletxe2x80x9d portion 216, some of which is also visible in the cross-section of FIG. 2b. Surface connector portion 210a is illustrated as including three portions exploded away from each other. Surface connector portion 210a includes an outer conductor or flange portion 210aoc, an inner conductor portion 210aic, and a cylindrical insulator portion 210ai. As illustrated, outer conductor 210aoc includes a tubular body portion 210aocb, a lower flange 210aoclf, and a strengthening flange 210aocsf. Body 210aocb of outer conductor 210aoc defines an inner bore 210aocbb which is dimensioned to accept the outer diameter of cylindrical insulator portion 210ai. Cylindrical insulator portion 210ai contains an axial bore dimensioned to accommodate the body portion of center or inner conductor portion 210aic. Assembly of surface connector portion 210a to printed circuit board 214a is accomplished by soldering or otherwise metallurgically connecting an end of center conductor portion 210aic to the center pad 214acp. The bore of insulator 210ai is then fitted over the protruding center conductor, the outer conductor 210aoc is fitted over and around insulator 210ai, and the lower flange 210aoclf is then soldered to that portion of the near ground plane 214agn surrounding aperture 214aga defined by dashed circle 214ac. 
Connector portion 210b of connector 18a of FIG. 2a is similar to portion 210a, and is similarly affixed to printed circuit board 214b. More particularly, the end of inner conductor 210bic is soldered or otherwise metallurgically connected to a pad (not designated) connected to the end of conductive trace 214bct so that it projects away from the surface of the board 214b. Dielectric insulator 210bi is fitted over protruding inner conductor 210bic. Outer conductor 210boc is then fitted over insulator 210bi with its lower flange abutting that portion of near ground plane 214bgn surrounding aperture 214bga. 
With both surface connector portions 210a and 210b of connector 18a fastened to their respective printed circuit boards 214a and 214b, the surface connector portions are aligned with axis 8, and brought together with bullet portion 216 of connector 18a therebetween. Bullet portion 216 includes a plurality of elongated conductive strips 216oc1, 216oc2, . . . fastened about a generally cylindrical dielectric body 216 with their axes of elongation coaxial with the local axis (which in the illustration is axis 8) and held in place by a spring ring 216ocr. The inner conductor is generally similar to the outer conductor. When the bullet portion 216 of connector 18a is pressed against the assembled surface portions 210a and 210b, the ends of the center conductor of bullet portion 216 spreads slightly to fit over the two center pins 210aic and 210bic, and the two ends of the outer conductor made up of pieces 216oc1, 216oc2, . . . contract slightly to fit within bore 210aocbb of the strengthening flange portions 210aocsf and of the equivalent bore associated with strengthening flange 210bocsf. In this fashion, the GPO connector set 18a of FIGS. 2a and 2b provides for a suitable transmission line interconnecting the two mutually parallel printed circuit boards 214a and 214b. 
It will be understood that each printed-circuit board 214a and 214b of FIG. 2a may be mounted within its own protective housing or module, with the surface mounting portions 210a and 210b projecting through one or more sides of the housing in the manner suggested in FIG. 1, and with the bullet portion of each connector as a separate item which is assembled to one or both sides of the module pair to be interconnected.
While the GPO connectors provide good connection between conductors on mutually parallel boards. they have disadvantages including relatively high cost. The assembly of each connector imposes an additional cost. A connector arrangement with the potential for lower cost would be advantageous.
An assemblage according to an aspect of the invention includes a first stripline board including first and second ground planes, and a circuit trace lying between the first and second ground planes. The first ground plane defines a circular first electromagnetic coupling aperture, and a conductive center trace lies at the center of the first electromagnetic coupling aperture, to thereby define, in cross-section, a first coaxial transmission line structure. The center trace of the first coupling aperture connects by way of a plated-through via to an end of the circuit trace of the first stripline board. A second stripline board includes first and second ground planes, and a circuit trace lying between the first and second ground planes of the second stripline board. The first ground plane of the second stripline board defines a circular second electromagnetic coupling aperture and a conductive center trace at the center of the second electromagnetic coupling aperture, to thereby define, in cross-section, a second coaxial transmission line essentially identical to the first coaxial transmission line. The center trace of the second coupling aperture connects by way of a plated-through via to an end of the circuit trace of the second stripline board. A separator defines two parallel planar surfaces spaced from each other by a predetermined distance. The separator defines a circular through aperture extending between the parallel planar surfaces. The through aperture is larger in diameter than the first and second coupling apertures, and the separator is physically fastened to the first and second stripline boards so that the through aperture is concentric with the first and second electromagnetic coupling apertures. An axially compliant third coaxial transmission line insert includes a center conductor coaxial with an outer conductor. At least one of the center conductor and the outer conductor of the third coaxial transmission line is in the form of a conductive ladder-shaped spring structure defining a plurality of rungs lying between elongated parallel members, which elongated members are formed into curved shapes resembling circles, or at least portions thereof, so that the rungs lie generally parallel with each other and with the axis of the other one of the center conductor and the outer conductor. The axially compliant third coaxial transmission line lies within the through aperture with a first end of the center conductor abutting the center trace of the first coaxial transmission line and a second end of the center conductor abutting the center trace of the second coaxial transmission line, and with a first end of the outer conductor of the coaxial third transmission line abutting the first ground plane of the first stripline board in a region surrounding the through aperture of the first stripline board, and with a second end of the outer conductor of the coaxial third transmission line abutting the first ground plane of the second stripline board in a region surrounding the through aperture of the second stripline board.
In a particularly advantageous avatar of the invention, both the center conductor and the outer conductor of the third coaxial transmission line are in the form of a conductive ladder-shaped spring structure defining a plurality of rungs lying between elongated parallel members, with the elongated members formed into curved structures so that the rungs lie generally parallel with each other and with the axis of the other one of the center conductor and the outer conductor. In such an arrangement, the elongated portions of the planar structures which are formed into circles may overlap so that more than a complete xe2x80x9cturnxe2x80x9d of structure is formed, or may not quite overlap, so that a xe2x80x9cgapxe2x80x9d lacking one or more rungs is left in one side of the structure. In this particularly advantageous avatar, the rungs of either the center conductor or the outer conductor of the third coaxial transmission line may bulge away from the axis of third coaxial transmission line at regions remote from the elongated members, to thereby form a somewhat barrel shape.
In a particular version of the structure, a generally barrel-shaped dielectric form may occupy the interstice between the center and outer conductors, and in a mode of making the connections, the dielectric form may have the ladder of the outer conductor wound thereabout. This dielectric form is a solid, but may be an elastomeric solid. The conductors of the ladders may comprise beryllium copper.
A method according to an aspect of the invention is for interconnecting transmission paths defined on the surface of first and second separated stripline boards, where the transmission paths are in the form of a circular aperture defined in a conductive ground plane of the associated stripline board, together with a conductive trace located at the center of the aperture. The method includes the step of making a center conductor having a nominal diameter by forming a first electrically conductive planar ladder-shaped spring structure defining two elongated parallel elements and a plurality of rungs extending therebetween into a tube-like form defined about an axis, so that the rungs lie parallel with the axis. The method includes the forming of a second electrically conductive planar ladder-shaped spring structure defining two elongated parallel elements and a plurality of rungs extending therebetween into a second tube-like form defined about an axis, so that the rungs lie parallel with the axis, to thereby define an outer conductor having a nominal diameter greater than the nominal diameter of the center conductor. The center conductor is placed coaxially within the outer conductor to thereby form a coaxial transmission line. The first and second stripline boards are juxtaposed with the circular apertures facing each other, and with the center and outer conductors of one end of the coaxial transmission line abutting the central trace and that portion of the ground plane surrounding the circular aperture, respectively, of the transmission path of the first stripline board, and with the center and outer conductors of the other end of the coaxial transmission line abutting the central trace and that portion of the ground plane surrounding the circular aperture, respectively, of the transmission path of the second stripline board, to thereby establish a coaxial transmission path between the transmission paths of the first and second stripline boards. The first and second stripline boards are then fastened together, including by way of intermediary structures, with the coaxial transmission path in place.